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Cite This: ACS Appl. Energy Mater. 2018, 1, 5887−5895
Physical and Chemical Defects in WO3 Thin Films and Their Impact on Photoelectrochemical Water Splitting Y. Zhao,† S. Balasubramanyam,‡ R. Sinha,† R. Lavrijsen,§ M. A. Verheijen,‡ A. A. Bol,‡ and A. Bieberle-Hütter*,†
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†
Electrochemical Materials and Interfaces (EMI), Dutch Institute for Fundamental Energy Research (DIFFER), 5600 HH Eindhoven, The Netherlands ‡ Plasma and Materials Processing (PMP), Department of Applied Physics, Eindhoven University of Technology (TU/e), 5600 MB Eindhoven, The Netherlands § Physics of Nanostructures and Center for NanoMaterials (cNM), Department of Applied Physics, Eindhoven University of Technology (TU/e), 5600 MB Eindhoven, The Netherlands S Supporting Information *
ABSTRACT: We evaluate the impact of defects in WO3 thin films on the photoelectrochemical (PEC) properties during water splitting. We study physical defects, such as microsized holes or cracks, by two different deposition techniques: sputtering and atomic layer deposition (ALD). Chemical defects, such as oxygen vacancies, are tailored by different annealing atmospheres, i.e., air, N2, and O2. The results show that the physical defects inside the film increase the resistance for the charge transfer and also result in a higher recombination rate which inhibits the photocurrent generation. Chemical defects yield an increased adsorption of OH groups on the film surface and enhance the PEC efficiency. An excess amount of chemical defects can also inhibit the electron transfer, thus decreasing the photocurrent generation. In this study, the highest performance was obtained for WO3 films deposited by ALD and annealed in air, which have the fewest physical defects and an appropriate amount of oxygen vacancies. KEYWORDS: WO3, atomic layer deposition (ALD), sputtering, defects, electron transport, photoelectrochemical water splitting
1. INTRODUCTION Converting sunlight into chemical fuels is a promising pathway for the utilization of renewable energy, which could potentially ease the dependence on fossil fuels and contribute to resolving global warming.1−3 Hydrogen production from photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes has been widely studied since the first discovery in 1972.4 During the past decades, great effort has been devoted toward exploiting efficient and robust photoelectrode materials.5−9 As a photoanode material, tungsten oxide (WO3) has been strongly investigated due to its high electron mobility (∼12 cm2 V−1 s−1 at room temperature (RT)), suitable band gap (2.6−2.9 eV), and good chemical stability.10−13 Despite these promising features, the conversion efficiency of the reported WO3 photoanodes is still far below the theoretical maximum conversion efficiency of 6.3%.14−18 This is attributed to its low light harvesting efficiency and high electron−hole recombination rate. Several studies have focused on overcoming these issues, such as selective doping of metal and nonmetal semiconductors,19,20 addition of cocatalysts, like CoOx or Co-Pi, to improve the water oxidation kinetics,21,22 forming a heterojunction via introduction of another semi© 2018 American Chemical Society
conductor layer, like BiVO4, Bi2S3, and Cu2O, to enhance the electron transport properties,23−25 or using nano/microstructured substrates.26,27 Various thin film deposition techniques have been used to achieve improved film structures, including a hydrothermal method,28 sputtering,29 pulsed laser deposition (PLD),30 and atomic layer deposition (ALD).14 Another important aspect that has not received enough attention yet is that the quality of the thin film can significantly affect the PEC properties. During PEC water splitting, the photoexcited holes are transferred from the bulk to the surface of the WO3 film for water oxidation, while the electrons are transferred to the external circuit through the functional thin film. Thus, physical defects, such as microsized holes or cracks in the film, can impact the photocatalytic activity and charge transfer efficiency.11,14 It is known in PEC water splitting that open porosity can increase the conversion efficiency by increasing the specific surface area.11,31,32 However, the physical defects inside the film and the interface between the functional layer and the electron collecting layer can also Received: May 28, 2018 Accepted: October 30, 2018 Published: October 30, 2018 5887
DOI: 10.1021/acsaem.8b00849 ACS Appl. Energy Mater. 2018, 1, 5887−5895
Article
ACS Applied Energy Materials impede electron transport which is often not considered. Chemical defects, such as oxygen vacancies, play a sensitive role in photocatalysis as well.33−35 Oxygen vacancies in WO3 films can act, for example, as electron donor to enhance the PEC efficiency.12,36 In recent density functional theory (DFT) studies on the electrochemical activity of WO3 and Fe2O3, oxygen vacancies were found to strongly impact the overpotential.37,38 A reduction of the overpotential of 0.2 V was found for Fe2O3 (110) in ref 38, depending on the density of oxygen vacancies. The overpotential is a measure for the electrochemical activity and therefore has a direct impact on the performance. Hence, the performance can be tailored by the chemical defects. Systematic, experimental studies on the impact of oxygen vacancies related to the electrode processing have not been done so far. It is therefore important to relate the physical and chemical defects of the WO3 film to the PEC properties and from this to conclude how to design and fabricate high performing electrodes for PEC water splitting. To this aim, we systematically evaluate the impact of defects in WO3 thin films on the PEC properties during water splitting. We study the physical defects by comparing the properties of WO3 thin films deposited by sputtering and ALD, respectively. The chemical defects are evaluated by different annealing atmospheres (air, N2, and O2). By this, systematic insight into the relation between film quality and PEC performance of thin film photoelectrodes is gained which will guide design rules for high performing photoelectrodes.
Table 2. ALD Process Parameters for WO3 Deposition
2.1. WO3 Film Deposition. WO3 thin films were deposited by reactive radio frequency (RF) sputtering and by plasma-enhanced ALD on commercial fluorine-doped tin oxide (FTO) (about 400 nm) coated 1 mm thick glass substrates (FTO15, Pilkington). Before the deposition, the substrates were cleaned following the procedure described in Malviya et al.39 Sputtered WO3 films were deposited by reactive RF sputtering (40/10 sccm Ar/O2 mixture, where the O2 is inserted at the sample position and the Ar at the W target) using a Bestec GmBH sputter tool from a 2 in. metallic tungsten target with a target−substrate distance of ∼100 mm. The main deposition parameters are listed in Table 1.
Table 1. Sputtering Process Parameters for WO3 Deposition parameter
value 100 W O2. The best performance was obtained from films annealed in air, which indicated an appropriate content of chemical defects. According to these results, it is worthwhile to relate the annealing conditions to the content of chemical defects. We can conclude that the selection of the thin film deposition process as well as the processing conditions, such as the annealing atmosphere, have a significant impact on the performance of the thin films toward water splitting activity. Electrodes should be designed and processed in order to minimize physical defects inside the functional films and at the interface between functional layer and electron conduction layer. This requires deposition techniques to deposit high quality, defect free thin films and interfaces. In addition, chemical defects should be well-tailored, since they can impact the performance in different ways. The obtained results should be transferred from plain thin films to nanostructured thin films in order to fabricate photoelectrodes with significant higher performance in the future.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 7. IMPS data of ALD WO3 films annealed in different atmospheres (applied potential, 1.23 V vs RHE; frequency range, 10−1−105 Hz).
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00849. 5893
DOI: 10.1021/acsaem.8b00849 ACS Appl. Energy Mater. 2018, 1, 5887−5895
Article
ACS Applied Energy Materials
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SEM images, AFM images, STEM images, photoluminescence (PL) spectra, photocurrent density−time curves, UV−vis light absorbance spectra, GIXRD patterns, XPS spectra, and comparison of PEC properties in ambient air and in synthetic air (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: A.Bieberle@differ.nl. ORCID
Y. Zhao: 0000-0003-4881-4751 A. A. Bol: 0000-0002-1259-6265 A. Bieberle-Hütter: 0000-0001-8794-9312 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.Z. and A.B.-H. acknowledge the financial support from NWO (FOM program 147 “CO2 neutral fuels”) and from the China Scholarship Council (CSC). The authors thank Nanolab at TU/e for access to SEM and XPS. We thank H. Genuit (DIFFER) for depositing thin films by sputtering, E. Zoethout (DIFFER) for the XPS measurements and results analysis, R. H. Godiksen for the PL measurements, and B. Barcones (TU/e) and R.G.R. Weemaes (Philips) for performing the FIB cross-sectioning. Solliance and the Dutch province of Noord Brabant are acknowledged for funding the TEM facility.
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on November 8, 2018, with an incorrect description of an apparatus in the Experimental Section. The corrected version was reposted on November 26, 2018.
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DOI: 10.1021/acsaem.8b00849 ACS Appl. Energy Mater. 2018, 1, 5887−5895
